ANALYTICAL KEY TO THE GENERA 229 



genera in which the genus he desires to identify is contained. After section 1 the key is 

 dichotomous. 



The illustrations in Fig. 3 are added to aid the verbal descriptions of section 1. They 

 do not depict any given species, but are merely diagrammatic representations of types 

 of structure. 



1. Cells discoid in valve view, rectangular in girdle view, pervalvar axis seldom more than 

 twice the diameter, valve structure arranged with reference to a central point, i.e. radiate 

 or concentric or nearly so (Figs, a, b, c) ... ... ... ... ... ... ... 2 



1. Cells cuneate in girdle view, segmental in valve view (Figs, d, e,f) .. . ... ... ... 25 



1. Cells tubular; pervalvar axis 4-20 times as long as the diameter; valve margin indefinite; 

 valves circular or oval, usually hyaline; connective zone well developed, composed of 

 annular segments or imbricate scales (Figs, g, h, i) ... ... ... ... ... ... 27 



1. Cells angular in valve view, 2 or more angles; girdle view not narrowly rectangular, 

 showing projections in the angles or corners (Figs, j, k, I, m) ... ... ... ... 33 



1. Cells straight or sigmoid, with more or less bilateral symmetry about the apical axis in the 

 valvar plane; valves linear, linear-lanceolate to broadly oval; girdle view rectangular 

 (Fig. q); median area bearing a raphe, i.e. a true cleft in the apical axis, also central and 

 polar nodules; valve markings arranged either at right angles to the raphe or subradiate 

 about the central nodule (Figs. n,o,p, q) ... ... ... ... ... ... ... 44 



1. Cells bearing raphe upon one valve and a pseudoraphe on the other (Figs, r, s) ... ... 53 



1. Cells with a pseudoraphe upon both valves ... ... ... ... ... ... ... 55 



1. Cells with neither raphe nor pseudoraphe; valves oval-lanceolate or linear-lanceolate; 

 valve surface bearing transverse bars (Fig. t) ... ... ... ... ... ... 63 



2. Valves with radial markings in the form of lines or furrows forming sectors ... ... 3 



2. Radial markings punctate or areolate, sectors not formed by lines or furrows ... ... 7 



3. Radial markings peripheral only, clearly not reaching the centre ... ... ... ... 4 



3. Radial markings originating at the centre ... ... ... ... ... ... ... 6 



4. Sectors alternately raised and depressed ... ... ... ... Actinoptvchiis (271) 



4. Sectors in one plane ... ... ... ... ... ... ... ... ... ... 5 



5. Radial markings furrowed, long; areolation concentric ... ... Arachnoidiscus (266) 



5. Radial markings furrowed, short; areolation radial ... ... ... Stictodiscus (265) 



6. Segments equal Asterolampra (267) 



6. Segments unequal Asteromphalus (269) 



7. Cells in colonies ... ... ... ... ... ... ... ... ... ... ... 8 



7. Cells solitary ... ... ... ... ... ... ... ... ... ... ... 18 



8. No direct contact between valve surfaces of adjacent cells; mucous communication 

 only 9 



8. Adjacent cells in contact, either by the valve surfaces or by horn-like processes from 

 them ... ... ... ... ... ... ... ... ... ... ... ... 11 



9. Cells aggregated to form mucous colonies of no definite shape ... Thalassiosira (237) 

 9. Cells united in chains by mucous threads ... ... ... ... ... ... ... 10 



10. Mucous threads single Thalassiosira (237) 



10. Mucous threads multiple Coscinoscira (*) 



11. Cells united by marginal spines... ... ... ... ... ... ... ... ... 12 



11. Cells united by valve surfaces ... ... ... ... ... ... ... ... ... 15 



* Coscinoscira, Detonula, Bacteriosira and Lithodesmium have been included to make the key 

 complete for all plankton genera. 



